Update "Try to avoid nullable fields" advice

[jiahao]

The current style guide says:

Try to avoid nullable fields
When using x::Union(Nothing,T), ask whether the option for x to be nothing is really necessary. Here are some alternatives to consider:

This section is probably outdated with the introduction of Nullable types (c.f. #8152).

[jiahao]

any thoughts @johnmyleswhite?

[johnmyleswhite]

I think this section should be almost entirely replaced: https://groups.google.com/d/msg/julia-dev/ygLhSERO25c/c-NOiKt8DxEJ

[nalimilan]

Actually I fixed that back in March via #10551.